This invention, made by the Institute of Nuclear Physics and Particle Physics at the Centre of Nuclear Research in Strasbourg,relates to the transport of electrostatic charges by mechanical means.
It is known that very high voltages can be obtained by means of electrostatic generators. To operate these generators, it is necessary to incorporate in them a device for conveying the electrostatic charges between a zone of low voltage (or earth) of the generator and the high voltage zone of the generator. This is known as the feed device of the electrostatic generator.
Electrostatic generator feed devices used at the present time are of two types:
The system of the Felici type using an insulating cylinder, which provides powerful outputs of (currents of) electrostatic charges without, however, being able to reach very high voltages; and
the system of the Van de Graaff type using a flat insulating belt, which can operate up to very high voltages but provides only limited outputs or charge currents.
The second system is used for supplying electrostatic accelerators emloyed mainly for research applications in nuclear physics. For other, more recent applications, it is desirable to be able to obtain both a high output or charge current and a very high voltage. This problem has not so far been resolved satisfactorily in Van de Graaff generators.
In these electrostatic accelerators, the distribution of electric field between the zone of very high voltage and the zone of very low voltage or earth voltage is controlled by separate devices such as gradient bars. The feed device, as well as other parts of the accelerator, are located in a gaseous atmosphere consisting mainly of sulphur hexafluoride. These gradient bars are placed on either side of each section of the belt, namely the ascending and descending section. The position of equilibrium of each belt section between the gradient bars surrounding them is, however, unstable. When attempts are made to increase the density of electric charges or, more precisely, the net balance of electric charges transported by each section of the belt, the belt is subject to mechanical instabilities which give rise to disturbances such as vibrations, premature wear and even breakdown.
The present invention seeks to provide a solution to the problem of increasing the charge output.
The proposed device for conveying the electrostatic charges is of the type comprising, inside a gaseous atmosphere:
A flat insulating belt turning in a closed circuit between an earthed zone and a high voltage zone which are spaced apart,
means for depositing electric charges on the belt in the earthed zone,
means for extracting electric charges from the belt in the high voltage zone and
two structures placed close to each section of the belt between the high voltage zone and the earthed zone to control the gradient of the electric field along the belt between these two zones.
According to a first feature of the invention, each of the structures placed close to each section of the belt extends continuously along the whole length of its particular section of belt and is non-conductive in the direction of displacement of the belt. This structure comprises, on the side facing the belt, a flat surface where it is designed to produce a cushion of ambient gas between itself and the belt.
In practice, the distance between the belt and the said flat surface is less than a millimeter.
According to another feature of the invention, each structure consists mainly of an epoxy resin equipped on the side of the belt with orifices placed at more or less regular intervals apart and supplied with ambient gas to produce a substantially uniform pressure at the orifice outlets. This enables the aforesaid air cushion to be produced.
The mean distance between adjacent orifices is advantageously several centimeters while the diameter of each orifice is of the order of a millimeter.
In a first embodiment of the invention the structure contains elongated conductive elements placed perpendicularly to the direction of movement of the belt and arranged at regular intervals along the belt and parallel to the plane of the belt.
These conductive elements may end flush with the surface of the insulating structure opposite the belt.
In one variation, the conductive elements are embedded at a selected depth within the volume of the insulating structure.
The feed tubules of the orifices advantageously extend at least in part to the inside of the said conductive elements.
In cross-section, the conductive elements are preferably so designed that they minimize the distortions of the local electric field created by their presence in the vicinity of the belt.
In another embodiment of the invention, the said structure is made of an insulating material which is homogeneous but rendered slightly conductive within its volume.
Inside the structure, the feed tubules of the orifices are advantageously inclined at an angle to the direction of movement of the belt. In the preceding embodiment, the portion of tubules extending through the insulating material is also advantageously set at an angle to the direction of movement.
According to another feature of the invention, the dielectric constant of the epoxy resin is approximately 5 to 8.
The two structures are preferably arranged each on the outside of its respective section of the belt.
The invention applies in particular to cases in which the ambient gas used is sulphur hexafluoride.
Other features and advantages of the invention will be apparent from the detailed description given below with reference to the attached drawings, as briefly described below.